If you receive rent via checks, make a photocopy or scan each one, every month.
Take a few seconds to compare the name from the current month’s check to the last one, and look for signs of a tenant in transition:
Joint check before, now a single name
A single name before, now joint
A different person altogether
These are all signs of a change, from divorce, to a new occupant in the unit, to new job status.
Check the phone number and add it to the file if it’s new.
Then, check the bank info and note any change in the file.
Occasionally, a tenant will submit a partial payment, or write something on the back where you endorse the check. Do not deposit or cash those checks before you talk to your lawyer. Under some circumstances, that could limit your rights to collect more from the tenant.